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Optimization of electrical energy production

The main focus in oil and gas offshore installations has been to optimize the production of oil and gas. There hasn’t been much focus on optimizing the electric energy production as well as consumption in order to increase the energy efficiency of the installation and also to reduce the emissions to the atmosphere. As the limits on the emissions to atmosphere are stricter, the existing oil and gas installations have to incorporate systems to optimize the energy efficiency and thus reduce the emissions to the atmosphere.

The maximum efficiency of electric energy production using the gas turbine technology is about 35 %, when the equipment is new and is operated at the optimal conditions. Over time, intelligent condition monitoring system has to be established in order to guarantee optimal electrical energy production and consumption, and reduced emissions.

There is a potential to increase the electrical energy production by using advanced control techniques. It is very common to have a decentralized control system for each gas turbine, but not an adaptive supervisory control system for the overall electrical energy production facility at the installation. The project focuses on developing an adaptive supervisory control system for the overall electrical energy production facility. An advanced condition monitoring system for the whole production facility, which collects critical information (performance characteristics in terms of efficiency and critical process variables such as pressure ratios, Turbine inlet temperature etc.) from each gas turbine system and provide the necessary information to the control system.

Waste heat recovery system

The flue gases from the gas turbines have considerable sensible energy which can be utilized to eliminate additional heating systems in the platform and further to produce electrical energy from the heat. On the offshore installations where there is no possibility of utilizing the heat from the gas turbines’ flue gases (for example, because of no installation of heat to heat capturing systems for the gas turbines) the thermal energy in the flue gases is not captured and being wasted. Thus the scope to produce electrical energy production from the energy available in the flue gases is significant, which is explained below with an example. The minimum temperature of the flue gas is around 200 to 300 ˚C, and this gives a theoretical additional total power efficiency of 10 -15%.

A normal power production on an installation can vary depending on the capacity and power requirements. For a typical platform, for example, if we assume the power requirement of 30 MWe, which is produced by gas turbine systems, with a typical efficiency up to 35%, which means a minimum of 30-40 % of the energy from the fuel is waste heat. If 12.5 % of this wasted energy can be transformed to electricity in a secondary thermodynamic cycle, which is equivalent to the order of 6-7 MW of electrical power system. This has a significant potential on reducing the carbon footprint!

The combined cycles (for example, Steam turbine) are not designed to produce electrical power at low temperatures (200 to 300 ˚C), and utilization of the thermoelectric effect based technologies give lower power efficiency thus make them difficult commercially competitive with other methods. However there exists several technologies that transform heat at 200 – 300 ˚C to electricity. These mature technologies above 120 ˚C are, for example, the Organic Rankine and the Stirling engine. However, there exists R&D challenges when the process is going offshore; the most important is floor space, weight, and HSE.

The most optimal electrical energy production can be achieved by combined optimization of the gas turbine systems and the waste heat recovery systems.

Organization of EOOS

The EOOS project consortium constisted of two universities, two research institutes, six technological companies and one oil & gas company. The project was partly funded by the Research Council of Norway, as an unse-driven innovation project in the Petromaks program.

The project started in 2010 and lasted until 2014 with a total budget of 28 MNOK, and was led by Dr. Rambabu Kandepu from Teknova AS.

Main Results

The project resulted in increased competency within waste heat recovery at low temperatures and advanced control system design with applications to offshore installations.Specifically, two gas turbines connected to one organic rankine cycle (ORC) results in an overall efficiency of 42.9% at full load, an improvement of 9.6 percentage points relative to two gas turbines without an ORC.